(1) Results from detailed equilibrium and kinetic studies conform to a mechanism by which multivalent immune complexes interact with Fc receptors (FcR) on P388D1 cells. The mechanism emphasizes the role of receptor density in governing the affinity with which a cell binds multivalent ligands. (2) A flow microfluorometric assay has been used to quantitate FcR on subsets of human cells. T gamma cells have a higher receptor density than monocytes, and the 3A1 hybridma antibody distinguishes T gamma cells from other subsets of FcR+ cells. (3) P388D1 cells internalize immune complexes rapidly at 37 degrees. FcR-mediated internalization also occurs for IgG-dimers, trimers, and perhaps monomers, but the rate of internalization increases with oligomer size. Subsequent to internalization, most FcR disappear from the cell surface. (4) Theoretical calculations on C1q binding to antibody coated targets predict that binding will be strongly dependent upon antibody density. Free IgG in the medium will inhibit Cl8 binding but only at low antibody density. (5) Antibody-coated ("franked") human peripheral blood lymphocytes specifically lyse tumor targets. Monocytes are not active in this system. Flow microfluorometric studies showed that antibody remained on the franked effector cell surface for long periods of time after franking.